1. Field of the invention
The present invention relates to a thin film transistor (TFT) substrate for a liquid crystal display (LCD).
2. Discussion of the Related Art
As information and telecommunication technologies advance, display devices for displaying images such as televisions, computer monitors, and notebook computers, etc., are increasingly used. Cathode ray tubes (CRTs) have been commonly used for the display devices. However, CRTs have the disadvantage of being heavy, and large.
Therefore, flat panel displays such as liquid crystal displays (LCD), plasma display panels (PDP), and an organic light emitting diodes (OLED), etc., are being used as substitutes for the CRTs. Of these, LCDs are commonly used with their advantages of high resolution, thin profile and the ability to be fabricated in a small or large size.
The LCDs are devices for displaying images by using electro-optical characteristics of liquid crystal molecules and can be divided into a twisted nematic (TN) mode LCD, a vertically aligned (VA) mode LCD, and an in-plane switching (IPS) mode LCD. The LCDs of each mode can display images by controlling a light transmittance by changing an arrangement of liquid crystal molecules according to an applied voltage.
In the various modes of LCDs, the IPS mode LCD receives much attention thanks to its advantageous characteristics that it can implement a wide viewing angle so that it has an excellent viewing angle at the side.
FIG. 1 is a plan view showing an IPS mode LCD according to the related art.
With reference to FIG. 1, the related art IPS mode LCD 1 includes a gate line, a data line, a unit pixel (P), and a liquid crystal layer.
The gate line and the data line cross each other. In this example, the gate line is formed in an x-axis direction and the data line is formed in a y-axis direction.
A plurality of sub-pixels are defined as gate lines and data lines cross each other.
The unit pixel (P) includes red, green, and blue sub-pixels (R, G, and B), and unit pixels (P) are disposed vertically and horizontally in the both directions along the gate lines and data lines.
A pixel electrode 50 and a common electrode 60 are formed in parallel to the data line in each of the red, green, and blue sub-pixels (R, G, and B) of the unit pixel. The common electrode 60 forms in in-plane field (E) together with the pixel electrode 50.
A liquid crystal layer is formed between a color filter substrate 20 and a TFT substrate 30 that are separated by a fixed gap and face each other. The color filter substrate 20 includes black matrixes and a plurality of color filters.
A liquid crystal layer is formed between the color filter substrate 20 and the TFT substrate 30. Liquid crystal molecules 10 in the liquid crystal layer have a dielectric constant anisotropy (Δε) and a refractive index anisotropy (Δn).
In FIG. 1, ‘C’ indicates a rubbing axis of an alignment film that determines an initial arrangement of the liquid crystal molecules 10.
In the IPS mode LCD 1, the liquid crystal molecules 10 are uniformly arranged along the rubbing axis (C) of the alignment film on the entire area of the LCD before the in-plane fields (E) are formed between the pixel electrodes 50 and the common electrodes 60.
When the in-plane fields (E) are formed between the pixel electrodes 50 and the common electrodes 60, the optical axes of the liquid crystal molecules 10 are uniformly rotated to be parallel to the in-plane fields (E) between the pixel electrodes 50 and the common electrodes 60.
Because of the characteristics of the liquid crystal molecules 10 that are uniformly arranged according to the formation of the in-plane fields (E) and have the refractive index anisotropy (Δn), when the LCD 1 is viewed at a certain tilt angle, it assumes an undesired color depending on a direction of a viewing angle. This will be described in detail with reference to equation (1):T=To*sin2(2χ)*sin2(π*Δnd/λ)  [Equation 1]wherein ‘T’ is light transmittance, ‘To’ is light transmittance with respect to the reference light, Δn is a refractive index anisotropy, ‘d’ is a cell gap, and λ is a wavelength of incident light.
According to Equation 1, when a phase difference Δnd changes, the wavelength of light incident to obtain a maximum light transmittance (T) changes. Thus, as the phase difference Δnd changes, the LCD 1 assumes an undesired color according to the direction of the viewing angle.
Namely, the refractive index anisotropy Δn of the liquid crystal molecules 10 differs in the direction (A) in which the longer axes of liquid crystal molecules 10 are viewed and in the direction (B) in which the shorter axes of the liquid crystal molecules are viewed, so the wavelength (λ) of light incident to obtain the maximum light transmittance (T) should change, and in this case, a color corresponding to the wavelength (λ) of incident light is manifested in the LCD 1.
In detail, because the refractive index anisotropy Δn relatively increases in the direction (A) in which the longer axes of the liquid crystal molecules 10 are viewed, the incident light wavelength (λ) is also relatively lengthened, resulting in that a yellowish color having a relatively long wavelength is displayed on the LCD 1.
Meanwhile, in the direction (B) in which the shorter axes of the liquid crystal molecules 10 are viewed, the refractive index anisotropy Δn relatively decreases, so the wavelength (λ) of light incident to reach the maximum light transmittance (T) also is reduced, resulting in that bluish color with a relatively short length is displayed on the LCD 1.
The phenomenon where the yellow or blue color is displayed at the certain azimuth angles is called a color shift that leads to degradation of picture quality of the LCD 1.
In order to prevent the color shift, a technique has been developed, in which each of the sub-pixels (R, G, and B) are divided into two symmetrical domains and the pixel electrodes 50 and the common electrodes 650 are symmetrically formed in the two domains to thus compensate for the difference of refractive index anisotropy Δn.
However, this technique has a problem where the liquid crystal molecules 10 at the boundaries of adjacent domains cannot be controlled, so that the boundary regions of adjacent domains must be overlapped with black matrixes that degrades an aperture ratio.